This invention relates to methods and systems for transmitting and receiving encoded data words and, more particularly, to a technique wherein intelligible information can be reproduced from such encoded data words even in the presence of an error condition which otherwise would distort or destroy the encoded information. A particular application of the present invention is to utilize digitally encoded data words, such as PCM data, for the recording of audio information on a video signal recording medium.
The recording of analog signals in the form of digitally encoded data is known. For example, in copending application Ser. No. 771,350, filed Feb. 23, 1977, now U.S. Pat. No. 4,138,694, and assigned to the assignee of the present invention, a technique is described wherein left and right channel audio signals are sampled and encoded in, for example, PCM form, and these PCM signals are recorded on video tape by a conventional video tape recorder (VTR). Since digital signals are recorded, this technique is susceptible to the inherent problem of drop-out wherein one or more bits of a data word or character may be distorted or obliterated because of microscopic defects in the magnetic tape or in the recording or playback operation. While such drop-out is relatively insignificant if it affects a bit of lesser significance, the reproduced data word can be seriously misrepresented or distorted if the drop-out affects a more significant bit. This problem is particularly noticeable if drop-out is present for a number of data words.
Data distortion or destruction due to drop-out also is found in the transmission of digital data over a communication channel, such as by radio communication, wire communication and the like. Another phenomenon which affects both digitial communication as well as digital recording and reproduction is the presence of an extended error which may exist over a significant number of data words or characters. This error, referred to herein as a burst error, may distort or destroy a relatively large number of data words. As a consequence thereof, the data which was represented by such data words cannot be recovered. If the digital data is intended to represent audio information, such as left and right channel stereo information, this burst error results in serious distortion of such stereo information. When the distorted digital signals are reproduced as audio signals, the distortion in sound is particularly noticeable, and undesirable "clicks" generally are reproduced.
The problem of drop-out which may affect only a single bit in a data word can be minimized by providing a parity bit for that word. For example, for odd parity, if the total number of binary 1's in a data word is odd, then the parity bit may be a binary 0. Conversely, if the total number of binary 1's in the data word is even, then the parity bit may be a binary 1. Analogously, a suitable parity bit may be provided for even parity. Of course, this simple expedient of a parity bit will not account for the drop-out of a plurality of data bits, nor will the parity bit be capable of compensating for burst errors.
To account for the problem of multiple bit drop-out and the problem of burst error in the communication of digital words or characters, various error correcting codes have been developed. For example, a data character may be constituted by a plurality of information bits which constitute a data word followed by a plurality of error correcting bits which may constitute an error correcting word. When this character is received, the error correcting code is decoded and used to determine whether the bits of the data word are in agreement with the error correcting word. That is, the data word is checked to determine if it is an allowable word with which the error correcting word is associated. If an error is detected, suitable steps may be taken for correction, such as re-transmission of the character. In accordance with another technique, error correction is attained by modifying the data word in an attempt to correct it so that it agrees with the received error correcting word. This correction is carried out on the basis of the pattern of disagreements of data words with their associated error correcting words.
Unfortunately, the typical use of an error correcting code word in association with a data word may not be capable of detecting or correcting burst errors of significant length. For example, if a data word is formed of m bits and the error correcting word is formed of n bits, then a burst error which exceeds some predetermined fraction of n might not be detected or corrected. If the burst error extends over some number 1 of characters, then 1 data words cannot be faithfully reproduced, and the information represented thereby is lost. When the data words represent audio information, such as stereo signals, this lost information substantially distorts the audio signals which ultimately are reproduced.